The invention relates to a method for dispensing adhesive on a circuit-board carrier member and a circuit-board produced by such method.
Circuit-boards for high-frequency use are known which comprise a carrier or some sort of housing member and, mounted thereon, a number of separate components, in particular substrates and/or electronic (mainly active) components. In microwave applications such electronic components may include MMICs. These components are secured to the carrier/housing via an electrically conductive adhesive. The adhesive acts to create electrical continuity between a metallised ground area on the substrate and the grounded carrier. Such an arrangement is shown in FIG. 1, in which two substrates 11, 12 are fixed by adhesive layers 13, 14 to a common carrier 15. Deposited on the two substrates are respective 50 xcexa9 lines 16, 17 which terminate in respective multiple bond-pad sites 18, 19. Linking opposite pairs of sites are wire-bonds 20, which may have been mounted using the well-known ball-and-wedge or wedge-and-wedge bonding techniques.
The wire bonds possess an intrinsic inductance which, in conjunction with a parasitic capacitance associated with the pads 18, 19, gives rise to a low-pass filtering effect 22 on any signal carried by the lines. In order to ensure that the corner-frequency of such a filter is sufficiently and reproducably high relative to the highest frequency of interest in the line signals, the wire-bonds are normally all made of the same length throughout the circuit-board (constant-wire-length (CWL) technique) and the pads are dimensioned and spaced such as to give rise to a low stray capacitance.
In order to ensure that the stray capacitance between the bond-pads 18, 19 and the carrier 15 is sufficiently low, it is advisable to strive for a termination of the adhesive layer at the substrate edge 21 which is as abrupt as possible. Should the adhesive find its way into the gap between the substrates or, worse still, end up covering one or both of the end-faces 22 of the substrates and/or well up into the gap, then the parasitic capacitance increases and the corner-frequency of the low-pass filter decreases, with a consequent deleterious effect on performance. This effect is illustrated in FIG. 2. Here the adhesive can be seen to have welled up into the gap 30, thereby creating new stray capacitance 31 between the bond-pads 18, 19 and the carrier 15.
The conventional way of applying adhesive to the carrier is to spread the adhesive in one of a number of patterns, namely stripes, spots, star-shapes, crosses, double-crosses, etc. One of the commonest patterns is the simple stripe, as shown in FIG. 3. It will be noted that, where each line of adhesive is begun and ended, there is an accumulation of adhesive 35. Similarly, where the adhesive is applied in patterns other than a straight-line pattern, accumulations can also occur at other discontinuities, namely at those points at which the direction of travel of the adhesive is changed. An example of this is a 90xc2x0 bend, in which accumulations occur at the ends of the angled line and also at the right-angle bend point.
Use of this technique in connection with two adjacent substrates linked by wire-bonds is illustrated in FIGS. 4a and 4b. Here the substrates (or MMICs, for example) are pressed onto the adhesive stripes, which spread slightly under the pressure, thereby partly filling in the inter-stripe gaps, and the adhesive is then allowed to set. Before this occurs, however, the build-ups 35 of adhesive at the end-discontinuities are subject to a squeezing action outwards, as illustrated in FIG. 4b, which then results in adhesive finding its way into the inter-substrate gap 30, as shown also in FIG. 2. Thus, while this known technique satisfies one design requirement, namely that the conductive adhesive should reach as far up to the edge of the substrate as possible in order to provide the best possible grounding of the substrate, it does not satisfy the second requirement, which is that adhesive should not find its way into the intersubstrate gap, thereby increasing stray capacitance.
In accordance with a first aspect of the invention there is provided a method for dispensing adhesive on a circuit-board carrier member as recited in claim 1 and under a second aspect of the invention there is provided a circuit-board having two components secured to a carrier member by an adhesive as recited in claim 8. Advantageous embodiments of the invention are contained in the sub-claims.